Moving around the desert Southwest, we see a lot of folds. This view is near the Valley of the Gods in Utah. Can you see all of the types of folds mentioned below? All of the folds mentioned in the text below are found in the arid Southwestern U.S.

Folds

Deep within the crust, as plates collide, rocks crumple into
folds
. You can model these folds by placing your hands on opposite edges of a piece of cloth and pushing your hands together. Your hands moving toward each other creates compressive stress. In sedimentary rocks, you can easily trace the folding of the layers (
Figure
below
). In the image below, the rock layers are no longer horizontal. They tilt downhill from right to left in a monocline. Once rocks are folded, they do not return to their original shape.

This is a geologic cross section of the Grand Staircase in Utah. Where the rocks fold up, at the left of the diagram, is a small syncline.

There are three types of folds: monoclines, anticlines, and synclines.

Monocline

A
monocline
is a simple “one step“ bend in the rock layers (
Figure
below
). In a monocline, the oldest rocks are at the bottom, and the youngest are at the top.

The rock layers in the center left are tilted in one direction, forming a monocline.

Anticline

An
anticline
is a fold that arches upward. The rocks dip away from the center of the fold (
Figure
below
). The oldest rocks are found at the center of an anticline. The youngest rocks are draped over them at the top of the structure. When upward folding rocks form a circular structure, that structure is called a
dome
. If the top of the dome is eroded off, the oldest rocks are exposed at the center.

(A) An anticline is a convex upward fold. (B) This anticline is at Calico Ghost Town, California.

Syncline

A
syncline
is a fold that bends downward (
Figure
below
). In a syncline, the youngest rocks are at the center. The oldest rocks are at the outside edges. When rocks bend downward in a circular structure, it is called a
basin
. If the rocks are eroded, the youngest rocks are at the center. Basins can be enormous, like the Michigan Basin.

(A) A syncline is a concave downward fold. (B) This syncline is seen at Calico Ghost Town near Barstow, California.

Vocabulary

anticline
: Fold that arches upward; older rocks are in the center, and younger rocks are at the outside.

basin
: Circular syncline; oldest rocks are in the center, and the youngest are on the outside.

dome
: Circular anticline; oldest rocks are in the center, and the youngest are on the outside.

fold
: Bend in a set of rocks caused by compression.

monocline
: Bend in a set of rocks that causes them to be inclined relative to the horizontal.

syncline
: Fold in the rocks that bends downward; the youngest rocks are at the center.

Summary

Rocks deform by compressive stress into folds.

A monocline is a simple bend in one-direction.

In an anticline, rocks arch upward. A three-dimensional anticline is a dome.

In a syncline, rocks arch downward. A three-dimensional syncline is a basin.